The present invention relates generally to threaded screw fasteners, and more particularly to a new and improved threaded screw fastener which has multiple thread structures. More particularly, the present invention a new and improved threaded screw fastener that has enhanced pull-out and back-out resistance characteristics and is readily manufactured.
Threaded fasteners well known in the art. In one application, threaded fasteners are used to fasten or secure waterproof or environmental membranes to underlying roof decking assemblies. A typical fastener includes a shank portion upon which a plurality of buttress-type threads are formed. The conventional buttress-type threads are formed from a slightly inclined rearward flank surface and a significantly inclined forward flank surface. The threads are formed having a predetermined thread pitch, as measured between the same points of successive thread crest portions, and a predetermined spacing as determined between the root region of the rearward flank surface of a particular thread and the root region of the forward flank surface of the next or successive thread.
It is well-known in the art that the rear flank surface, as well as the pitch, play critical roles in, or effectively determine, the pull-out resistance and anti-back-out characteristics of the fastener, while the forward flank surface, and the spacing, likewise play critical roles in, or effectively determine, the installation or insertion torque characteristics of the fastener.
In order to increase the pull-out and back-out resistant characteristics of the fasteners, a thread structure has been used in which each thread of the fastener includes rearward and forward flank surfaces which effectively serve to enhance the pull-out resistance characteristics or properties of the threaded screw fastener.
While this back-out resistant fastener works well for its intended purpose, it has been found to be quite difficult to manufacture. That is, the cutting die that is used to cut the threads in the shank are quickly worn, requiring frequent replacement and maintenance. As such, the quantity of fasteners that can be fabricated limits the useful life of the cutting die and further limits the efficiency of the manufacturing process.
Accordingly, there is a need for a fastener having high back-out resistant characteristics and is readily manufactured. Desirably, such a fastener has enhanced pull-out resistance and increased manufacturability without adversely impacting the installation characteristics of the fastener.